1. Field
One embodiment of the present invention relates to a disk drive that employs an assisted writing method using a high-frequency magnetic field.
2. Description of the Related Art
In the field of disk drives, a representative example of which is a hard disk drive, magneto-resistive (MR) elements and giant magneto-resistive effect (GMR) elements have recently been put to practical use as read-head elements for use in magnetic heads. The MR element and the GRM element have remarkably increased the recording capacity of the disk drive. Further, the ultra-high recording density in the disk drive is promoted by utilizing perpendicular magnetic recording, because perpendicular magnetic recording can, in principle, achieve a higher recording density than longitudinal magnetic recording.
However, the ultra-high recording density cannot be easily promoted due to the thermal fluctuation that is inherent to magnetic recording. As a magnetic recording method that may solve this problem, there has been proposed a so-called high frequency assisted writing method that uses a high-frequency magnetic field. (See, for example, U.S. Pat. No. 6,011,664 and U.S. Patent Application Publication No. 2005/0207050.)
This method is a technique of applying a magnetic field of a frequency much higher than the recording-signal frequency to a prescribed tiny part of a magnetic disk (hereinafter referred to as disk), thereby reducing the coercive force (Hc1) that part has in the recording-signal frequency region to half (Hc2) or less.
At the time the coercive force of the disk is thus reduced, a magnetic head applies a recording magnetic field to said part of the disk. Thus, data can be magnetically recorded on a disk that has high magnetic anisotropy energy (Ku) and, therefore, can record data at a higher density.
Some prior-art references disclose a method of applying a high-frequency magnetic field. More precisely, a high-frequency current is supplied to a coil coupled to a magnetic pole, exciting the magnetic pole and causing the pole to generate a high-frequency magnetic field, and this magnetic field is applied to a disk. This method has a problem, however. The smaller the part of the medium in which to record data is in order to increase the recording density, the more drastically the magnetic field that can be applied to the part will decrease. Consequently, the coercive force can hardly be reduced at the recording part, rendering it difficult to accomplish high frequency assisted writing.
In order to solve this problem, a method has been proposed in which a spin torque oscillator (STO) is used as source of a high-frequency magnetic field. (See, for example, U.S. Patent Application Publication No. 2005/0023938 and U.S. Patent Application Publication No. 2005/0219771.) The STO has, for example, a GMR element or a TMR (Tunneling Magneto-Resistive effect) element. The operating principle of the STO is as follows. When a current is supplied to the STO, the spin of the electrons passing through the spin-injection layer is polarized. The stream of the electrons thus polarized exerts a spin torque to the oscillation layer, magnetizing the oscillation layer. Thus magnetized, the oscillation layer undergoes ferromagnetic resonance, generating a high-frequency magnetic field.
An example of the STO is the microwave oscillator disclosed in Xiaochun Zhu and Jian-Gang Zhu, “Bias-Field-Free Microwave Oscillation Driven by Perpendicularly Polarized Spin Current,” IEEE Transaction on Magnetics, Vol. 42, No. 10, October 2006.
The phenomenon that the oscillation layer generates a high-frequency magnetic field is prominent if the element size is equal to or less than tens of nanometers. Therefore, the area in which the high-frequency magnetic field emanating from the STO works is limited to within a radius of only tens of nanometers from the STO.
It is desired that the oscillation frequency of the STO be equal or nearly equal to the ferromagnetic resonance frequency of the recording layer of the disk. If the STO is arranged near the recording pole of the magnetic head and if the magnetic head is arranged close to and opposed to the disk, the high-frequency magnetic field generated by the STO can be applied to only the tiny recording part of the disk. As a result of this, only the coercive force at the tiny recording part can be reduced.
In the disk drive, the recording pole may apply a recording magnetic field to the recording part of the disk at the time the coercive force on the disk decreases. If the recording magnetic field is so applied, the recording part will undergo flux reversal, whereby data is written in the disk.
The STO consumes as little power and generate as little heat as the conventional GMG element or the conventional TMR element. In addition, any head that has an STO arranged near the recording pole can be manufactured in the same way as the conventional magnetic head. The manufacturing cost of the magnetic head having an STO would not increase so much. In view of this, the high frequency assisted writing method that uses an STO is promising as a magnetic recording method that may promote ultra-high recording density in disk drives.
However, the high frequency assisted writing method using an STO has been found to be disadvantage in the following respect.
In any magnetic head that has an STO, the oscillation layer of the STO is arranged near the recording pole. Inevitably, the leakage recording magnetic field emanating from the recording pole is applied to the oscillation layer, too. The oscillation frequency of the STO therefore fluctuates as the recording magnetic field changes. Consequently, the high-frequency magnetic field emanating from the STO destabilizes the reduction in the coercive force of the recording layer of the disk. This ultimately hinders stable, high-quality, high frequency assisted writing.
Hence, in order to achieve a uniform, stable high-density recording by using the high frequency assisted writing method, the oscillation frequency of the STO should be stable even if the recording magnetic field emanating from the recording pole changes.